1. Field of the Invention
This invention relates to a magnetic toner to be used in electrophotography, electrostatic photography, magnetic recording or electrostatic printing.
2. Description of the Prior Art
Toners (particularly of one-component type) have generally been produced in desired particle sizes by fusion-mixing homogenously with a thermoplastic resin to be homogeneously dispersed therein, and then crushing by a pulverizing device and being classified by a classifying machine. This crushing method is capable of producing considerably excellent toners, but it has certain kinds of restrictions. That is, the toner obtained by use of the crushing method should be made of a material which is brittle to some extent so as to be readily crushed. However, if such a material is too brittle, excessive micropulverization may be caused to such an extent that fine powders must disadvantageously cut in order to obtain toners with an appropriate size distribution, which leads to an increase in cost. Besides, further micropulverization may sometimes occur in a developing vessel in a copying machine. In addition, when a low melting material is used for improvement of the heat fixing characteristic, fusion may occur in a crushing device or a classifying device, whereby continuous production of a toner may be made impossible.
Other necessary conditions for a toner is to have a triboelectrifying characteristic suitable for development, to form an excellent image not to deteriorate in performance on standing, not to cause agglomeration (blocking etc.), to have an appropriate heat or pressure fixing characteristic and to incur no contamination on the surface of a photosensitive material, and so on.
The toner for development of electrostatic charges manufactured by suspension polymerization overcomes the drawbacks of the crushing method. That is, absence of the crushing step requires no brittleness, and the spherical forms obtained give excellent free flowing property and therefore uniform in triboelectrification. Further, by appropriate control of polymerization and by use of a crosslinking agent, there can be obtained a toner excellent in heat fixing characteristic.
In these days, there is a trend of shifting from two-component type toners to one-component type toners, and magnetic toners are most popular among the one-component type toners in practical applications. It is very effective to prepare a magnetic toner by suspension polymerization.
However, when toners containing magnetic materials are to be prepared by suspension polymerization, there sometimes ensue problems in dispersion of the magnetic material. In the crushing method, a binder resin, magnetic powders and additives therefor are kneaded at a high viscosity under a high mechanical shear by a roll mill or a kneader, whereby very good dispersion state can be obtained. However, in case of a polymerization method, a monomer, a magnetic material and others are mixed under a low mechanical shear such as by means of a homomixer, whereby the dispersion is obviously insufficient, resulting in lower image density developed than the one developed by using the toner of the crushing method. To solve such problems, the magnetic material employed must be well dispersed in the synthetic resin monomer used. It should not inhibit the polymerization. If the magnetic powders are excessively fine, they can difficulty be dispersed, giving rise to localization of magnetic powders, whereby the magnetic powders cannot be evenly distributed among the suspended particles at equal proportions, and further the particle size of the toners produced by the suspension polymerization are liable to be distributed broad. Hence, toner properties such as free flowing property, tribo-distribution, image forming characteristics, etc. are considerably worsened. Thus, it is necessary to improve the dispersibility of the magnetic powder. However, the great surface area of magnetic powders requires a large amount of a dispersant to be used, which incurs such unfavorable influences on the toner particles such as decrease in resistance of the toner, etc.
As another disadvantage, when the magnetic powders are too large in size, the number of magnetic powder particles to be incorporated in each suspended particle may become extremely small, with some particles containing substantially no magnetic powder. In such a case, the toner particles formed have magnetic characteristics which greatly vary among particles to result in different developing characteristics of individual toners, leading to lowering in developing performance as a whole.
Further, even when magnetic powders are of certain appropriate sizes, too large specific area of magnetic powders may retard polymerization, lowering the productivity in the end. If an amount of the initiator is increased for removing this drawback, polymer with sufficiently high molecular weight cannot be obtained, which makes it difficult to obtain a toner with appropriate thermal characteristics.